Rinnai 25
Ducted Gas Heater IM
8. DUCTING AND OUTLETS
8.1 DUCT DESIGN AND SIZING
Good duct design and sizing are essential to every Central Heating system. Use the Product Sizing Guide and
technical data within this manual for the best results and follow these guidelines:
• Ductwork should be well insulated and airtight and have a minimum insulation rating of R1.0 (R1.5 in some
areas). Ensure that ducting complies with the Building Code of Australia.
• The ducting should be well fastened to pops, BTOs, outlet boots and neck adapters adequately with duct tape,
in accordance with AS 4254, HB 276 and AS5141.
• It should also be properly sized, and curves and bends should be smooth enough to ensure that the air flows
through efficiently, quietly and with minimal resistance.
• The registers and diffusers should be large enough and of good design. They should minimise noise, while
providing the correct distribution pattern.
• The positive return air system should be fitted with a grille large enough to accept the full air capacity of the
system at low noise levels.
• If the system uses high level outlets (e.g. ceiling diffusers), then the return air inlet should be at a low level.
Ceiling systems with a high level return air may result in reduced performance.
• For SP4UN heaters, access to the ductwork must be provided for general maintenance and service to the
supply air thermistor sensor.
NOTE
It is important that the ducting should be well insulated. It is mandatory under building codes to
install insulated, fire rated duct.
8.2 RETURN AIR GRILLE
If a filter is fitted to the return air grille, make sure it is easily accessible for regular cleaning.
Table 5 gives the minimum recommended return air grille sizes for each model heater.
Table 5. Minimum Recommended Return Air Grille Selection
Without Filter
With Filter
Model
Grille Size
(m²)
Example Size
(mm)
Grille Size
(m²)
Example Size
(mm)
SP415UN, SP421UN, BX520
0.26
(400x650)
0.39
(400x1000)
SP435UN, BX526
0.36
(400x900)
0.54
(400x1350)
SP430UN
0.31
(400x800)
0.47
(400x1200)
NOTE
Sizes are based on maximum airflow for typical 'egg-crate' grilles. For all other types, refer to the
manufacturer’s specifications.
Example:
If a grille has a free ventilation opening of 500mm x 800mm, the grille size is 0.5m x 0.8m = 0.4m²
With a filter, this grille suits the SP415UN, SP421UN and BX520 heaters, which require at least 0.39m².
8.3 RETURN AIR POP CONFIGURATION CHANGES
Some heater models offer an alternative for the return air pop from the side position to the end of the cabinet (for
example, to accommodate a lay-down kit). Connecting the return air pop to the end of the unit reduces the rated air
flow by up to 30%.
IMPORTANT
RETURN AIR CONNECTION AT THE END OF THE UNIT - REAR ENTRY (F3)
Connecting the return air duct to the end of the unit reduces the rated airflow by up to 30%.
For ADD-ON cooling applications,
MAKE SURE
that the minimum recommended airflow
is maintained. Refer to the ADD-ON installation manual for more information.
